Flexible circuit board

ABSTRACT

A flexible printed circuit board (FPCB), which is applied to various electronic display devices, may include a base, a first metal layer and a second metal layer on both surfaces of the base, a first plating layer on the first metal layer, a second plating layer on the second metal layer, and a first insulating pattern and a second insulating pattern respectively disposed on some region of the first plating layer and the second plating layer, wherein the first plating layer and the second plating layer may have different thicknesses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of U.S. patentapplication Ser. No. 16/688,476 filed Nov. 19, 2019 (now U.S. Pat. No.11,202,367), which is a Continuation Application of U.S. applicationSer. No. 15/765,308, filed Apr. 2, 2018 (now U.S. Pat. No. 10,517,172),which is a U.S. National Stage Application under 35 U.S.C. § 371 of PCTApplication No. PCT/KR2016/010300, filed Sep. 12, 2016, which claimspriority to Korean Patent Application No. 10-2015-0140252, filed Oct. 6,2015, whose entire disclosures are hereby incorporated by reference.

BACKGROUND 1. Field

An embodiment of the present invention relates to a flexible printedcircuit board (FPCB), which is applied to various electronic displaydevices.

2. Background

A chip on film (COF) base applied to an electronic device such as a flatpanel display such as an LCD or a mobile device performs a function ofallowing a circuit chip and a circuit wiring to be disposed in a regionwhere bending is performed due to flexible characteristics. Such a COFbase may increase a degree of freedom in a design of electronic devicesand is widely used for various shapes and structures of bent electronicdevices.

However, in the COF base, cracks may occur in a process of a repeatedbending action or coupling in a bent state of a circuit pattern formedon a base film, or a problem that a metal pattern layer may be brokendue to a tensile force generated at the time of bending the circuitpattern may occur.

Referring to FIG. 1 , FIG. 1 is a view of an example of application to aproduct in which a COF base is used. As shown in FIG. 1 , a COF base 3is used to constitute a device including a display panel 1. That is, theCOF base 3 may function to electrically connect the display panel 1 anda FPCB 2, and may be bent and connected as shown in FIG. 1 in order tosecure a space inside a device. In this case, an IC chip 4 may befurther mounted on the COF base 3.

In such a structure, recently, as the display panel requires a highresolution, a number of channels required for the COF base 3 isincreasing. Accordingly, there is an increase in demand for adouble-sided COF having circuit wiring patterns on both sides of aflexible board, not a conventional single-sided COF base.

In the case of the double-sided COF, the circuit wiring pattern shouldbe thinly formed on the both sides of the flexible board, andaccordingly, cracks due to bending, may occur in a portion 3 a where thebending is performed when the double-sided COF base constitutes a deviceincluding the display.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 is a conceptual diagram illustrating an example of a device towhich a general COF is applied.

FIG. 2 is a view of a general double-sided flexible printed circuitboard (FPCB).

FIG. 3 is a conceptual cross-sectional view illustrating a structure ofa FPCB according to an embodiment of the present invention.

FIG. 4 is a process conceptual diagram illustrating a manufacturingprocess of a FPCB according to an embodiment of the present invention.

DETAILED DESCRIPTION

Hereinafter, a configuration and action according to the presentinvention will be described in detail with reference to accompanyingdrawings. In the following description with reference to theaccompanying drawings, the same components are designated by the samereference numerals regardless of the reference numerals, and redundantdescription thereof will be omitted. Although terms such as first,second, etc. can be used to describe various components, theabove-mentioned components should not be limited by the above terms. Theterms are only used to distinguish one component from another.

FIG. 2 is a comparative view for comparison with a flexible printedcircuit board (FPCB) according to an embodiment of the presentinvention, and is a conceptual diagram of a structure of which a platinglayer is formed on a double-sided FPCB, and FIG. 3 is a conceptualcross-sectional view illustrating a structure of a FPCB according to anembodiment of the present invention.

Referring to FIG. 2 , the FPCB according to a structure of FIG. 2 , isan example of a structure in which a metal layer 20 is disposed on bothsurfaces of a base 10 at a center portion, solder-resist layers 50 and60 are respectively implemented on an upper portion thereof, and platinglayers 30 and 40 having a uniform thickness are further laminated on themetal layer 20. In such a structure of FIG. 2 , when the FPCB is bentwith center on a bending reference line X formed on one side of thesolder-resist layers 50 and 60, tensile strengths of the plating layerand the metal layer disposed at upper and lower portions act differentlyfrom each other, so that cracks easily occur.

Referring to FIG. 3 , a FPCB 100 according to an embodiment of thepresent invention may include a base 110, a first metal layer 122 and asecond metal layer 124 on both surfaces of the base 110, and a firstplating layer 130 disposed on the first metal layer 122 in a laminatedstructure and a second plating layer 140 formed on the second metallayer 124. In particular, in this case, there is provided the FPCB,including a first insulating pattern 150 and a second insulating pattern160 respectively disposed on some region of the first plating layer 130and the second plating layer 140, wherein the first plating layer 130and the second plating layer 140 have a structure of regions withdifferent thicknesses.

That is, in the structure of the FPCB according to an embodiment of thepresent invention, the first metal layer 122 and the second metal layer124 constituting a circuit pattern on the base 110 are provided, and aplating layer is implemented thereon in order to improve signalcharacteristics and protect the circuit pattern through a laminatingprocess such as a plating. A thickness of the plating layer formed on anupper portion surface of the metal layer, that is, the thickness of thefirst plating layer 130 may be different from that of the second platinglayer 140 on a lower portion surface. A thickness of some region of thefirst plating layer 130 may be implemented thicker than that of thesecond plating layer 140. Accordingly, later, when the FPCB is mountedon an electronic device in a bent structure, the thickness of theplating layer may be implemented thicker with reference to the boundaryline X at which the bending is performed. That, is, a stiffness is addedto a portion where a change of a tensile force is relatively small, anda degree of stiffness is reduced on an opposite side where a change of atensile force is relatively large, so that a change in a tensile forceof the circuit pattern may be buffered as a whole. Therefore, generationof cracks may be reduced remarkably in the first metal layer and thesecond metal layer implementing the circuit pattern as well as the firstplating layer and the second plating layer.

Further, as in the structure of FIG. 3 , in an embodiment of the presentinvention, the insulating patterns 150 and 160 for protecting patternsmay be disposed on surfaces of each of the first and second platinglayers 130 and 140. In this case, the first insulating pattern 150 andthe second insulating pattern 160 respectively disposed on some regionof the first plating layer 130 and the second plating layer 140, may bedisposed on only some region of the first plating layer 130 and thesecond plating layer 140 which are to be partially exposed. Inparticular, the first insulating pattern 150 and the second insulatingpattern 160 are disposed in a range not overlapped with the boundaryline X where the bending is performed, so that cracks of the insulatingpattern at the time of bending may be prevented.

Furthermore, the first insulating pattern 150 may be implemented in astructure buried at a predetermined depth from a surface of the firstplating layer 130. That is, a side surface portion of the firstinsulating pattern 150 may be in contact with a side surface portion ofthe first plating layer 130. A predetermined portion of the firstinsulating pattern 150 may be implemented in a structure buried in thefirst plating layer 130. Accordingly, the FPCB according to anembodiment may have structural stability. Meanwhile, the FPCB accordingto an embodiment may expand ductility of the insulating pattern at thetime of bending to a predetermined portion at the boundary line X wherethe bending is performed, so that a stress may be buffered. Therefore,generation of cracks may be prevented in the circuit pattern and theplating pattern disposed on an upper portion surface thereof. Inaddition, the first insulating pattern 150 and the second insulatingpattern 160 may be disposed at a portion 3 a where the bending isperformed at the time of constituting a device. Accordingly, generationof cracks caused by bending of the FPCB may be reduced.

Specifically, the first plating layer 130 according to an embodiment ofthe present invention may be composed of a first region B correspondingto a lower portion of the first insulating pattern 150 and a secondregions A and C which are regions other than the region where the firstinsulating pattern 150 is disposed. The first region B may be a regionwhere bending is performed. For example, according to an embodiment,when the FPCB is used to constitute a device including a display panel,a cross section of the first region B may have a curved shape. The firstregion B may refer to a region where the upper surface (a portion of theupper surface) of the base and the upper surface (another portion of theupper surface) are bent and face each other, or the lower surface (aportion of the lower surface) of the base and the lower surface (anotherportion of the lower surface) face each other.

The second regions A and C may be regions other than the region wherebending is performed. For example, when the FPCB according to anembodiment is used to constitute a device including a display panel, across section of the second regions A and C may have a straight lineshape. The second regions A and C may include regions where the base ispartially bent to connect with a display panel or a separate circuitboard. That is, the second regions A and C may be regions where onesurface (a portion of one surface) of the board and one surface (anotherportion of one surface) do not face each other.

A thickness of the first plating layer 130 in the second regions A and Cmay be greater than that of the first plating layer 130 in the firstregion B. Accordingly, the first insulating pattern 150 may have astructure buried in the first plating layer 130 in the first region B.Therefore, the first insulating pattern 150 may buffer a tensile forceby structural characteristics, and may control a difference in a tensileforce applied to upper and lower portion surfaces of the base, therebyreducing generation of cracks.

In addition, as shown in FIG. 3 , the first plating layer 130 accordingto an embodiment of the present invention may be distinguished into afirst sub-plating layer 132 in direct contact with the first metal layer122 and a second sub-plating layer 134 having a thickness that allowsthe first insulating pattern 150 to be buried therein. In particular, inthe case of the second sub-plating layer 134, the second sub-platinglayer 134 may be implemented with substantially the same thickness asthat of the second plating layer 140 disposed on a lower portion surfaceof the base 110.

The first sub-plating layer 132 may include an alloy structure due toaction occurring on the surface of the first metal layer 122. Inaddition, the second plating layer 140 may include an alloy structuredue to action occurring on the surface of the second metal layer 124.

For example, when the first metal layer 122 and the second metal layer124 are implemented with Cu and the first sub-plating layer 132 and thesecond plating layer 140 are plated with Sn, the first sub-plating layer132 and the second plating layer 140 may be implemented with a structureincluding a Cu/Sn-based alloy by a chemical action at a laminatinginterface. That is, a material of a portion corresponding to a thicknessof the first region of the first plating layer and a material of thesecond plating layer may be implemented as the same to each other.

On the other hand, materials of the first sub-plating layer 132 and thesecond sub-plating layer 134 may be implemented differently from eachother. That is, in an embodiment of the present invention, it isadvantageous for convenience of the process that the first plating layerand the second plating layer are plated with the same material, but, inorder to implement a unique structure of the present invention,materials of the first sub-plating layer 132 and the second sub-platinglayer 134 may be implemented differently from each other in the processof plating the first plating layer twice. More specifically, the firstsub-plating layer and the second sub-plating layer may have differentcontents of the same alloy material with each other.

As described above, the difference between the materials of the firstsub-plating layer and the second sub-plating layer is implementedthrough the following process. In the manufacturing process of FIG. 3 ,after the first metal layer and the second metal layer are formed of aCu material, in a structure in which the first sub-plating layer 132 isplated with Sn, the first insulating pattern 150 is coated, the secondsub-plating layer 134 and the second plating layer 140 are plated, andthe second insulating pattern 160 is coated, when a heat treatmentprocess including thermal curing is performed, a diffusing action of Cuand Sn occurs. In particular, in this case, a part of Sn of the firstsub-plating layer 132 and the second sub-plating layer 134 and a part ofCu of the first metal layer 122 may be diffused to form an alloy. Aconcentration of diffusion of Cu may be continuously lowered as toward asurface of the second sub-plating layer 134 from the first sub-platinglayer 132, and a concentration of diffusion of Sn may be lowered astoward the first metal layer 122 from the surface of the secondsub-plating layer 134.

In addition, a part of Sn of the second plating layer 140 and a part ofCu of the second metal layer 124 may be diffused to form an alloy, aconcentration of diffusion of Cu may be continuously lowered as toward asurface of the second plating layer 140 from the second metal layer 124,and a concentration of diffusion of Sn may be lowered as toward thesecond metal layer 124 from the surface of the second plating layer 140.That is, a content of a material of each layer is different. Anelectrochemical migration resistance may be prevented by such adiffusion phenomenon of Cu/Sn, and thus a short-circuit defect due tometal growth can be prevented.

A thickness of the second sub-plating layer 134 corresponding to thesecond regions A and C of the first plating layer 130 may besubstantially the same as that of the second plating layer 140.Alternatively, a thickness of the first sub-plating layer 132corresponding to the first region B of the first plating layer 130 maybe substantially the same as that of the second plating layer 140.

That is, a thickness and a shape of the first plating layer may beimplemented by distinctiveness of the process of the FPCB according toan embodiment of the present invention.

In the structure of the FPCB, the first insulating pattern 150 and thesecond insulating pattern 160 may be implemented as a structure in whichthe first insulating pattern 150 and the second insulating pattern 160are disposed to face each other with reference to the first region. Morespecifically, the first insulating pattern 150 and the second insulatingpattern 160 may be disposed at positions symmetrical to each other onupper and lower portions of the base 110, or may be disposed as astructure in which the first insulating pattern 150 and the secondinsulating pattern 160 are partially overlapped with each other. Such astructure may act as a factor capable of adjusting generation of cracksby controlling a tensile force of the FPCB. In an embodiment of thepresent invention, it is preferable that the first insulating pattern150 and the second insulating pattern 160 should be disposed within arange not exceeding the reference of the boundary line X of the FPCB.

In addition, since the base 110 according to an embodiment of thepresent invention is in contact with an acid or the like during etching,the base 110 having chemical resistance which is not eroded by suchchemicals and heat resistance which does not deteriorate by heatingduring coupling, may be used. As an example of resin forming such a basemay include glass epoxy, bismaleimide-triazine (BT) resin, polyester,polyamide, polyimide or the like. In particular, in an embodiment of thepresent invention, it is preferable to use a film made of polyimide. Asan example of the polyimide film composing the base may be listed allaromatic polyamides synthesized with pyromellitic dianhydride andaromatic diamine, and all aromatic polyamides having biphenyl skeletonsynthesized with biphenyltetracarboxylic dianhydride and aromaticdiamine. In particular, in an embodiment of the present invention, allthe aromatic polyamides having a biphenyl skeleton may be used.

The first metal layer and the second metal layer may be implemented invarious metal material layers made of a conductive metal, and in anembodiment of the present invention, any one of an electrolytic copperfoil and a rolled copper foil laminated on the base may be used. Theelectrolytic copper foil may form a circuit pattern in a fine pitch. Inan embodiment of the present invention, the first metal layer and thesecond metal layer have a comprehensive concept of a structure includingvarious circuit patterns and wiring patterns implemented by patterning(e.g., photolithograph process, etc.) a copper foil layer in addition toa simple metal layer.

FIG. 4 shows an example of a manufacturing process of a FPCB accordingto an embodiment of the present invention as described above in FIG. 3 .

Referring to FIG. 4 , according to an embodiment of the presentinvention, first, in the process of manufacturing the FPCB of (a), afirst sub-plating layer 132 may be formed on an upper portion surface ofstructures in which a first metal layer 122 and a second metal layer 124are implemented on both sides of a base 110 via plating.

The base 110 may include a flexible plastic. For example, the base 110may be a base composed of a polymeric material layer such as polyimide(PI), polyethylene terephthalate (PET), or polyethylene naphthalate(PEN), and in the present embodiment, it will be described as an examplethat a sheet layer using polyimide (PI) is applied to one embodiment. Inthe present embodiment, a case in which a thickness of the base 110 isimplemented in the range of 12.5 to 125 μm may be applied.

In addition, the first metal layer 122 and the second metal layer 124may be implemented in a circuit pattern implemented on the base 110, andmay have a thickness within the range of 8 to 9 μm. Further, the firstmetal layer 122 and the second metal layer may be any one of Cu and Al,or various metal layers and alloy layers having conductivity may beused. For example, of course, the first metal layer 122 and the secondmetal layer 124 may contain at least one metal among copper (Cu),aluminum (Al), chromium (Cr), nickel (Ni), silver (Ag), molybdenum (Mo),gold (Au), titanium (Ti), and alloys thereof. The thickness of the firstmetal layer 122 and the thickness of the second metal layer 124 may beimplemented to be substantially equal to each other, and the thicknessthereof may be implemented in the range of 1 to 20 μm.

Further, the first sub-plating layer 132 may be composed of an Sn-platedlayer, or may be composed of any one of an Ni/Au alloy, electrolessnickel immersion gold (ENIG), Ni/Pd and organic solderabilitypreservative (OSP). In this case, a thickness of the first sub-platinglayer 132 may be implemented to be 0.1 μm or less.

Next, in the process of (b), a first insulating pattern 150 isimplemented on an upper surface of the first sub-plating layer 132. Thefirst insulating pattern 150 may contain an insulating material. Thefirst insulating pattern 150 may be a resist layer. For example, thefirst insulating patter 150 may be a solder resist layer containing anorganic polymer material. For example, the first insulating pattern 150may be implemented in the range of 1 to 20 μm by printing an insulatingpattern using a solder resist ink or by applying various materials(cover-lay, polymeric material) having insulation characteristics. Asdescribed above, the first insulating pattern 150 is a structure inwhich a part of a surface of the first sub-plating layer 132 is exposed,and may be implemented in only some region.

Then, in the process (c), both upper and lower surfaces of the structureimplemented in the process (b) are plated. Through this, the FPCB isimplemented in a structure in which a second sub-plating layer 134 islaminated on the upper portion surface of the first sub-plating layer132, and a second plating layer 140 is formed in contact with the secondmetal layer 124. In this case, thicknesses of the second sub-platinglayer and the second plating layer may be implemented to be 1 μm orless. In particular, when the first sub-plating layer 132 and the secondsub-plating layer 134 are plated with the same material, the firstplating layer may be implemented as one layer. However, as describedabove, in the case of the first sub-plating layer 132 and the secondplating layer 140, a predetermined alloy may be formed by interactionbetween the first metal layer 122 and the second metal layer 124. Inaddition, the first insulating pattern 150 may be implemented in astructure in which a part of the first insulating pattern 150 is buriedin the first plating layer 130 by the plating process. The upper surfaceof the first insulating pattern 150 may be disposed higher than theupper surface of the second sub-plating layer 134. Accordingly, a sidesurface of the first insulating pattern 150 may be partially in contactwith the second sub-plating layer 134. That is, the side surface of thefirst insulating pattern 150 may be in contact with the secondsub-plating layer 134 in a region corresponding to the thickness of thesecond sub-plating layer 134.

Next, in the process (d), a second insulating pattern 160 may bedisposed on an upper surface of the second plating layer 140. The secondinsulating pattern 160 may contain an insulating material. The secondinsulating pattern 160 may be a resist layer. For example, the secondinsulating pattern 160 may be a solder resist layer containing anorganic polymer material. For example, the second insulating pattern 160may be implemented in a range of 1 to 20 μm by printing an insulatingpattern using a solder resist ink or by applying various materials(cover-lay, polymeric material) having insulation characteristics. Thesecond insulating pattern 160 may be disposed at a position where thesecond insulating pattern 160 and the first insulating pattern 150 areoverlapped with each other with center on the base 110. More preferably,it has been described above that the second insulating pattern 160 andthe first insulating pattern 150 may be disposed at a positionsymmetrical to each other. In particular, in an embodiment of thepresent invention, when the insulating pattern is implemented by theabove-described solder resist, a thermal curing process or a dryingprocess may be added.

By this process, the first sub-plating layer 132 and the second platinglayer 140 may be made of a material having a predetermined alloystructure by action occurring on the surfaces of the first metal layer122 and the second metal layer 124, and each layer may be classifiedinto material-changed layers according to a content of alloy in eachlayer by using an X-ray diffraction method and an AES analysis method.

Hereinafter, results of comparative experiments on bendingcharacteristics of the structure of FIG. 2 and the structure accordingto an embodiment of the present invention will be described.

The FPCB having the structure of FIG. 2 and the FPCB having thestructure of FIG. 3 were formed in the same size. Next, repeatedoperations of performing bending with center on the bending referenceline X as shown in FIGS. 2 and 3 were performed. At this point, thefirst and second metal layers were formed of Cu, the first and secondplating layers were formed of Sn, and the first and second insulatingpatterns were formed of solder resist.

Based on a configuration of FIG. 3 , a polyimide film having a thicknessof 35 μm was applied to the base, the first metal layer and the secondmetal layer were formed of a Cu layer of 8 μm, the first sub-platinglayer was formed of Sn layer of 0.05 μm, the second sub-plating layerwas formed of Sn layer of 0.4 μm, and the first plating layer is formedto a thickness of 0.45 μm. Further, the second plating layer was formedof Sn of 0.4 μm. Further, the insulating pattern was formed of a solderresist layer having a thickness of 10 μm.

In addition, in the structure of the comparative example shown in FIG. 2, the base 10 was implemented with the polyimide film having the samethickness of 35 μm, and the metal layer 20 on the base was formed of aCu layer of 8 μm, the plating layers 30 and 40 were formed to have athickness of 0.45 μm, the insulating pattern 50 and 60 was formed tohave a thickness of 10 μm, which were the same standards.

In the case of the structure shown in FIG. 2 , cracks were not observeduntil 10 times of bending, but cracks were generated from 10 times ofbending. Accordingly, it was confirmed that reliability of the FPCB ofthe structure according to the comparative example was lowered.

In the case of the structure according to an embodiment of FIG. 3 , itwas confirmed that cracks were not generated even at 40 times ofbending. It was confirmed that micro cracks were generated when thenumber of bending was about 50 times. That is, it was confirmed thatbending characteristics of the embodiment were improved by about 500%compared with the comparative example of FIG. 2 .

This is, as described above, due to a structure in which a difference intensile force between the upper and lower portions occurs, it clearlyshows that the structure in which the tensile force is controlled by thestructure according to an embodiment of the present invention is moreeffective in preventing generation of cracks.

Further, when the structure of FIG. 3 is designed with the samenumerical values as in the experimental example, pure tin (Sn) of thesecond sub-plating layer may be distributed to a range of 0.1 um from asurface of the second sub-plating layer.

Two plating layers may be disposed on at least one surface of thedouble-sided FPCB. For example, the first sub-plating layer 132 may bedisposed under the first insulating pattern 150, and the secondsub-plating layer 134 may be disposed on a side surface of the firstinsulating pattern 150. The first insulating pattern 150 according to anembodiment may have a buried structure in which the first insulatingpattern 150 is surrounded by the second sub-plating layer 134 on thefirst sub-plating layer 132, and thus tensile force may be relieved atthe time of bending. Accordingly, an embodiment may prevent cracks orde-filming of the metal layer and/or the plating layer, therebyimproving electrical reliability of the FPCB.

In addition, the insulating pattern according to an embodiment mayincrease an area which is in contact with the plating layer, therebypreventing separation of the insulating pattern. Accordingly,reliability of the FPCB according to an embodiment may be improved.

In addition, in the FPCB according to an embodiment, since the secondsub-plating layer 134 is partially disposed on the first metal layer122, generation of metal particles, for example Sn particles, which aregenerated in the plating process is reduced, thereby improvingreliability of the FPCB and a COF module including the same.

Specific embodiments have been described in the detailed description ofthe present invention as described above. However, various modificationsare possible within a scope of the present invention. The technical ideaof the present invention should not be limited to the describedembodiments of the present invention but should be determined by theclaims and equivalents thereof.

Embodiments of the present invention is directed to solving theabove-described problems, and a flexible printed circuit board (FPCB)which may be applied to a bent coupling structure is implemented, inparticular, a thickness of a plating layer formed on each of circuitpatterns of upper and lower surfaces is made different, and a positionof a protective layer for protecting the plating layer is implemented ina buried structure, so that a crack phenomenon may be prevented, whichis caused by a change in a tensile force at the time of bending and alsogeneration of particles may be reduced remarkably, which is generated ina plating process, thereby providing the FPCB capable of enhancingreliability of a product.

In particular, it is possible to eliminate a problem that a circuitwiring pattern formed on the FPCB is damaged by cracks and thedouble-sided COF base does not play its original role.

Technical Solution

As a means for solving the above-described problems, according to anembodiment of the present invention, there is provided a flexibleprinted circuit board (FPCB), including a base, a first metal layer anda second metal layer on both surfaces of the base, a first plating layeron the first metal layer, a second plating layer on the second metallayer, and a first insulating pattern and a second insulating patternrespectively disposed on some region of the first plating layer and thesecond plating layer, wherein the first plating layer and the secondplating layer may have different thicknesses.

Advantageous Effects

According to an embodiment of the present invention, a flexible printedcircuit board (FPCB) which may be applied to a bent coupling structureis implemented, in particular, a thicknesses of a plating layer formedon each of circuit patterns of upper and lower surfaces is madedifferent, and a position of a protective layer for protecting theplating layer is implemented in a buried structure, so that a crackphenomenon may be prevented, which is caused by a change in a tensileforce at the time of bending and also generation of particles may bereduced remarkably, which is generated in a plating process, therebybeing an effect capable of enhancing reliability of a product.

Any reference in this specification to “one embodiment,” “anembodiment,” “example embodiment,” etc., means that a particularfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention. Theappearances of such phrases in various places in the specification arenot necessarily all referring to the same embodiment. Further, when aparticular feature, structure, or characteristic is described inconnection with any embodiment, it is submitted that it is within thepurview of one skilled in the art to effect such feature, structure, orcharacteristic in connection with other ones of the embodiments.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the spirit and scope of the principles ofthis disclosure. More particularly, various variations and modificationsare possible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

What is claimed is:
 1. A flexible printed circuit board (FPCB),comprising: a base; a first metal layer and a second metal layer onrespective surfaces of the base; a first plating layer on the firstmetal layer; a second plating layer under the second metal layer; afirst insulating pattern provided on the first plating layer; and asecond insulating pattern under a part of the second plating layer,wherein a thickness of the first plating layer between the firstinsulating pattern and the base is less than a thickness of the secondplating layer between the second insulating pattern and the base, andwherein an upper surface of the first plating layer has a step portion,and a lower surface of the second plating layer does not have a stepportion.
 2. The FPCB of claim 1, wherein the first plating layerincludes: a first region corresponding to a lower portion of the firstinsulating pattern; and a second region which differs from the firstregion, wherein the second plating layer includes: a third regioncorresponding to an upper portion of the second insulating pattern; anda fourth region which differs from the third region, wherein the firstplating layer includes: a first sub-plating layer; and a secondsub-plating layer on a first part of the first sub-plating layer,wherein the first insulating pattern is provided on a second part of thefirst sub-plating layer, and wherein a thickness of the second region ofthe first plating layer is greater than a thickness of the first regionof the first plating layer.
 3. The FPCB of claim 2, wherein a thicknessof the third region of the second plating layer corresponds to athickness of the fourth region of the second plating layer.
 4. The FPCBof claim 2, wherein a step difference between an upper surface of thesecond sub-plating layer and an upper surface of the first insulatingpattern is smaller than a step difference between a lower surface of thesecond plating layer and a lower surface of the second insulatingpattern.
 5. The FPCB of claim 2, wherein the first sub-plating layer andthe second sub-plating layer include different materials.
 6. The FPCB ofclaim 5, wherein the first sub-plating layer and the second sub-platinglayer have different contents of tin (Sn).
 7. The FPCB of claim 2,wherein the first sub-plating layer and the second plating layer includea material of an alloy structure with the first metal layer and thesecond metal layer on surfaces of the first metal layer and the secondmetal layer.
 8. The FPCB of claim 1, wherein the base is a chip on film(COF) base, and the first insulating pattern has a structure buried inthe first plating layer.
 9. The FPCB of claim 1, wherein a width of thefirst insulating pattern is different from a width of the secondinsulating pattern.
 10. The FPCB of claim 1, wherein a first distancebetween the first insulating pattern and the base is different from asecond distance between the second insulating pattern and the base. 11.The FPCB of claim 10, wherein the first distance is smaller than thesecond distance.
 12. The FPCB of claim 1, wherein an upper surface ofthe first insulating pattern is positioned at a higher level than anupper surface of the first plating layer.
 13. The FPCB of claim 1,wherein a lower surface of the second insulating pattern is positionedat a lower level than a lower surface of the second plating layer. 14.The FPCB of claim 1, wherein the first insulating pattern is overlappedwith the second insulating pattern in a vertical direction of the base.15. The FPCB of claim 1, wherein the first plating layer is thinner thanthe first insulating pattern, and wherein the second plating layer isthinner than the second insulating pattern.
 16. The FPCB of claim 1,wherein a thickness of at least a part of the first plating layer issmaller than a thickness of the second plating layer.
 17. A displaydevice comprising the FPCB of claim 1, wherein the first insulatingpattern is provided at a portion in a bent structure where a change oftensile force is relatively large, and wherein the second insulatingpattern is provided at another portion in the bent structure where achange of tensile force is relatively small.